How are you using APIs in your ecommerce or vintage clothing projects?

I’m working on a small ecommerce project around vintage clothing and secondhand fashion, and I’m starting to explore different APIs to make things smoother (for example, handling payments, currency conversion, and maybe even shipping or notifications). I’m still fairly new to structuring everything properly, so I’m curious how others here are using APIs in their own projects.

Which APIs or integrations have you found most useful for small online stores, and are there any common mistakes or pitfalls I should avoid when I’m just getting started?